The clearance between the stationary seals of a combustion turbine or compressor turbine and the tips of the rotating blades therein must not be so great as to permit an excessive amount of fluid to pass there between, and thereby reducing the efficiency of the turbine. On the other hand, clearances cannot be too small because high centripetal loading and high temperatures may cause blades to lift or to grow radially. Such blade lifting or radial growth can cause blade tips to rub the stationary seal and may eventually cause seal and/or blade tip damage.
In addition, the differences in thermal response time of the various turbine components can result in the mechanical interference between stationary and moving parts under certain conditions. This is certainly the case during the restart of a hot turbine where contact between the compressor/turbine blades and the stationary blade ring has resulted in massive compressor and turbine damage. Even a slight rub will destroy blade seals and reduce the efficiency of a combustion turbine. The obvious solution is to prolong restart until the turbine cools, however, this may require a delay of many hours. The situation is further complicated by the competing need to spin-cool the turbine following shutdown to prevent sagging or humping of the rotor. Both can be done only if the blade clearance is accurately measured, and appropriate action is taken based upon this on-line measurement.
Capacitance blade clearance probes are used to study blade clearance patterns to establish restart and spin-cool rules. However, capacitance probes are sensitive to handling and require careful calibration prior to each use. In particular, capacitance probes must be kept clean to ensure that they function properly, making their use limited to testing applications in carefully controlled environments. Consequently, capacitance clearance probes have proven to be both inaccurate and unreliable for commercial on-line monitoring. Additionally, capacitance clearance probes have proven to be unreliable in turbo-machinery applications for testing at low speeds of revolution, such as at turning gear speeds.
A number of blade clearance systems have been developed for steam turbines, such as those described in U.S. Pat. No. 4,987,555. These systems depend upon indicia on the blades shroud to obtain a meaningful proximity measurement. However, the approaches do not appear readily applicable to combustion turbine applications.
It is known to use eddy current testing systems coupled to pulsed eddy current probes for detecting voids, cracks, and corrosion in metal objects, such as described in U.S. Pat. No. 6,037,768. Such systems are commercially available from SE Systems, Inc. under the trade designation SmartEddy™, and from Eddy Current Technologies, Inc. under the trade name Ectmachine™. However, these systems have not been adapted to rotating turbo-machine blade clearance measurement applications.